Osteochondroma

Osteochondroma is the most common benign bone tumour, accounting for 35% of all benign osseous lesions and 8-9% of all bone tumours. It is a developmental anomaly rather than a true neoplasm: a cartilage-capped projection of bone that grows by enchondral ossification while the skeleton grows, and stops when it matures.

Who. Peak age is 10-20 years, the years of active growth, with a male predominance of 2:1.

Where. A solitary lesion can occur in any bone formed by enchondral ossification and is rare in bones of membranous origin such as the skull vault. 70% arise in the metaphysis of a long bone, most often the distal femur, proximal tibia and proximal humerus. Axial lesions (spine, ribs, pelvis) have a higher malignant potential.

How many. 85% of cases are solitary, sporadic and non-hereditary. The other 15% are hereditary multiple exostoses (HME), caused by EXT1 or EXT2 mutations.

Natural history. The lesion enlarges during skeletal growth and stops when the physis closes. Any growth after physeal closure is highly suspicious for malignant transformation to secondary chondrosarcoma.

A developmental error. Osteochondroma arises from aberrant cartilage at the periphery of the growth plate, which herniates through a defect in the perichondral ring. The displaced cartilage keeps its growth potential and undergoes enchondral ossification, forming a cartilage-capped bony projection whose cortex and medullary cavity are continuous with those of the underlying bone.

The genetics of HME. Inheritance is autosomal dominant with high penetrance. Mutations lie in EXT1 on chromosome 8q24 or EXT2 on 11p11-13, genes that encode glycosyltransferases for heparan sulfate synthesis, and loss of heterozygosity is required for a lesion to develop.

What it means for the family. The transmission risk to offspring is 50%, so first-degree relatives of HME patients are screened and genetic counselling is important for family planning. The disease also disturbs growth, producing deformities of the forearm and lower leg.

Malignant transformation. Secondary chondrosarcoma develops in 1% of solitary osteochondromas and in 5-25% of HME cases, although the upper end of that range is disputed (see Controversies). Transformation typically occurs in adulthood and is heralded by renewed growth, increasing pain or an enlarging cartilage cap. The chondrosarcoma is usually low-grade (Grade 1), but wide excision is required for cure.

Gross. A cap of hyaline cartilage, covered by a fibrous perichondrium, sits on a bony stalk of normal trabecular bone and marrow that is continuous with the parent bone. The cap is typically 3-10 mm thick in adults; paediatric caps can be up to 20-30 mm during active growth.

Histology. In the benign cap, chondrocytes sit in lacunae, and enchondral ossification is active at the base of the cap during growth. Malignant change shows:

• Hypercellularity - increased chondrocyte density
• Nuclear atypia - enlarged, hyperchromatic nuclei
• Myxoid change - degeneration of the cartilage matrix
• Permeative growth - invasion into adjacent soft tissue

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Pedunculated. A narrow stalk carries a bulbous cap. Pedunculated lesions are more common in the metaphyses of long bones and point away from the adjacent joint, because muscle and tendon traction draws the growing stalk in that direction over years. They are easier to excise and cause less deformity. A lesion pointing toward the joint should make you question the diagnosis.

Sessile. A broad base attaches the lesion to the bone. Sessile lesions are more common in flat bones such as the pelvis and scapula, and they carry a slightly higher risk of malignant transformation because their cartilage caps are typically thicker.

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Skeletal Deformity in Multiple Osteochondromas (HME)

Beyond malignancy risk, the major morbidity of HME is growth-plate tethering deformity. The lesions tether physes asymmetrically, and deformities of the forearm and lower leg are the most common. The governing principle is guided growth (hemiepiphysiodesis) while the physes are open, and corrective osteotomy nearer maturity or where a deformity impairs function. HME children therefore need regular monitoring of alignment and limb length throughout growth, not just malignancy surveillance.

The characteristic deformities and their treatment:

• Forearm - ulnar shortening, radial bowing, radial head subluxation and reduced forearm rotation; managed according to Masada type by osteochondroma excision, ulnar lengthening, radial osteotomy or hemiepiphysiodesis
• Ankle valgus - from distal fibular shortening or distal tibial physeal involvement; medial distal-tibial hemiepiphysiodesis (guided growth) in the growing child, or supramalleolar osteotomy near maturity
• Genu valgum - medial distal-femoral or proximal-tibial guided growth, or osteotomy
• Coxa valga and acetabular dysplasia
• Limb-length discrepancy - guided growth or osteotomy, with early recognition and intervention

The forearm is the classic site and has its own named classification.

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History. The most common presentation is a painless mass, found incidentally, or an enlarging lump that parents notice in their child. Mechanical pain comes from impingement on adjacent structures or from a bursa over the lesion, a common reason these lesions become painful. A tendon or muscle irritated over the lesion may snap, and the stalk of a pedunculated lesion can fracture, which is rare.

Examination. The mass is firm, non-tender and fixed to bone, in the metaphyseal region of a long bone, and typically 1-10 cm across. Check for neurovascular compression and assess joint motion for a mechanical block.

Algorithm

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Observation

Observation is appropriate for:

• An asymptomatic lesion in a child or adolescent
• Small size with no neurovascular compromise
• Typical imaging (cortical continuity, thin cap)
• No growth after skeletal maturity

During growth. Children are examined clinically once a year, with a radiograph if symptoms develop or a change in size is noted.

At skeletal maturity. Once the growth plates have closed and a solitary lesion is stable and asymptomatic, no further routine follow-up is needed. The patient is discharged and told to return with new growth, pain or other new symptoms.

In HME. Lifelong clinical surveillance is recommended because of the higher malignancy risk: annual clinical examination for life, a baseline MRI of any large or symptomatic lesion, and a repeat MRI if growth or pain develops.

Surgical Excision

The indications, remembered as PNCC:

• Pain - mechanical, from impingement or bursa formation, not relieved by conservative measures
• Neurovascular compromise - compression of adjacent nerves or vessels
• Cosmesis - a deformity that concerns the patient or family
• Concern for malignancy - growth after maturity, a cap greater than 2cm, pain in an adult

The technique, in order:

• Approach - expose the lesion through an approach that protects neurovascular structures: direct lateral or medial for the femur and tibia, deltopectoral for the proximal humerus
• Excision - osteotomise at the junction of stalk and parent bone with an osteotome or oscillating saw, removing the entire cartilage cap with the base of the stalk and aiming for a 5mm margin of normal bone at the base
• Specimen - send the entire specimen to pathology intact and oriented; if malignancy is suspected, send the cap separately for immediate frozen section before closing the wound
• Closure - irrigate, close the periosteum if possible to prevent soft-tissue adhesion, and close the soft tissues in layers; a drain is not usually required for simple excisions

Find the nerve first. The giant radial osteochondroma shown below was excised through an extended lateral exposure, with elevation of the fascio-muscular flap, direct visualisation and protection of the posterior interosseous nerve, then reconstruction of the soft-tissue envelope. The operative priority is identifying the displaced nerve before the stalk is mobilised. Around the proximal fibula, plan for variant nerve displacement and expose the nerve before excision; a posteromedial fibular-head lesion can compress the tibial nerve.

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Management of Malignant Transformation

Confirmed secondary chondrosarcoma is treated by wide excision, with the margin and any adjuvant therapy set by grade.

Prognosis. Low-grade secondary chondrosarcoma has an excellent prognosis with wide excision, 90% 5-year survival. High-grade lesions have poorer outcomes and may require chemotherapy or radiotherapy, though chondrosarcoma is relatively chemo-resistant.

During surgery. Neurovascular injury risk depends on location, the common peroneal nerve at the proximal fibula and the axillary nerve at the proximal humerus, and is rare (under 1%) with careful dissection. A pedunculated stalk may fracture during manipulation, and incomplete excision of the cap leads to recurrence.

After surgery. Wound infection occurs at standard surgical-site rates (2-3%). Haematoma is rare and usually self-limiting. Pathological fracture through weakened bone at the excision site is rare, and more common with large sessile lesions.

Of the disease. Malignant transformation is the most serious complication, and the local ones sit alongside it in the table.

Vascular compression. New swelling, bruising, pulse change or ischaemic symptoms require urgent vascular imaging.

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Recurrence. Complete excision brings recurrence to nearly 0%. Recurrences typically occur within 2 years and may require re-excision.

Solitary lesions. The prognosis is excellent. Most lesions are asymptomatic and require no treatment, and those that are excised have excellent outcomes with low recurrence.

HME. Quality of life is impaired by multiple lesions, skeletal deformities of the forearm and lower leg, and lifelong malignancy surveillance.

Global Epidemiology

Society Guidance - Side by Side

Registries & Resource-Setting Variation

The cartilage cap threshold. The classic 2cm adult threshold is a pragmatic rule, not an absolute cut-off. Murphey's original archival data used 1.5cm, and later MRI series favour 2cm in adults. Caps change with technique (ultrasound or MRI) and slice plane, so serial change matters more than any single number, and growth after maturity outweighs a static cap measurement as a red flag.

The true malignancy risk in multiple osteochondromas (MO, synonymous with HME). Historical figures of up to 25% came from referral-centre cohorts with selection bias, and contemporary population-based estimates cluster around 1-5% lifetime risk. EXT1 genotype, pelvic or axial sites and large sessile lesions concentrate the risk. The right denominator, all MO patients or only those reaching sarcoma units, remains debated.

Surveillance strategy. No randomised evidence defines the optimal interval or imaging modality for MO surveillance. Whole-body MRI is attractive, but cost and access limit its routine use, and most centres rely on symptom-driven imaging plus periodic clinical review. Over-surveillance risks incidental findings and patient anxiety.

Atypical cartilaginous tumour. The line between grade 1 secondary chondrosarcoma and a benign reactive cap is genuinely difficult, and WHO now uses the term atypical cartilaginous tumour for some low-grade appendicular lesions. Imaging-pathology-clinical correlation, not histology alone, drives the call.